Low loss, polarization-independent, large bandwidth mode converter for edge coupling
A mode converter formed by parallel tapered waveguides on a SiN platform. The waveguides form a trident structure comprising a main waveguide with an inverse taper structure, and a pair of waveguides on each side of the main waveguide. Each adjacent waveguide has a taper structure but one that is opposed to that of the main waveguide, namely, a width that gradually increases along the direction of light propagation to a larger value at an end tip thereof. The end tips of the waveguides terminate along a common input/output facet of the converter. The adjacent waveguides help to shape the mode of the light propagating through the main waveguide, in so doing enabling the converter to exhibit high coupling efficiency and polarization independence in the full optical communication bands (i.e., from O to L-band) by successfully tuning the mode shape at a chip facet. The trident mode converter enables efficient optical fiber-to-chip coupling.
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This application relates generally to integrated optics and, in particular, to interfaces that facilitate edge coupling, e.g., chip-to-chip coupling, optical fiber-to-photonic chip coupling, and the like.
Brief Description of the Related ArtMany integrated photonic applications, such as radio-frequency signal processing and optical neural networks, require robust and low loss operation over polarization and wavelength. While integrated photonic components can be designed to meet these specifications, coupling light to and from a photonic integrated circuit (PIC) has been a difficult challenge. In particular, the difficulty lies in the large modal mismatch between an integrated waveguide and a standard single-mode (e.g., SMF-28) fiber, and this mismatch inhibits efficient optical input-output (IO) coupling.
Today, two main solutions exist to address this problem, namely, edge (also referred to as in-plane, butt or end-fire) coupling, and vertical (out-of-plane) coupling. A typical edge coupler (EC) has an inverse taper structure wherein the waveguide width gradually reduces along the direction of light propagation to a small value at an end tip. Edge coupling benefits from polarization independence and operation over large bandwidths, but the approach necessitates a large overlap between the fiber and waveguide optical modes. Further, a typical single inverted taper-based mode converter fails to simultaneously provide low coupling loss, polarization independence, and broad bandwidth operation. For instance, an inverted taper coupler for silicon-on-insulator ridge waveguide can achieve ˜0.36 dB and ˜0.66 dB coupling losses for TM and TE waveguide modes, respectively, but only over a working bandwidth of 80 nm. Vertical coupling, on the other hand, redirects the light out of the waveguide plane, typically through the use of diffractive gratings. Diffractive gratings, however, suffer from high polarization and wavelength sensitivity, leading to lower coupling efficiency.
There remains a need to provide for improved optical fiber-to-PIC interfaces.
BRIEF SUMMARYA mode converter formed by three (3) parallel tapered waveguides on a SiN platform is configured for low loss, polarization-independent and broad bandwidth coupling of light to an optical fiber. The waveguides form a trident structure comprising a main, central waveguide with an inverse taper structure (i.e., wherein the waveguide width gradually reduces along the direction of light propagation to a small value at an end tip), and preferably an adjacent waveguide positioned on each respective side of the main, central waveguide. Each adjacent waveguide has a taper structure but one that is opposed to that of the main waveguide. In particular, each adjacent waveguide has a width that gradually increases along the direction of light propagation to a larger value at an end tip. The end tips of the three parallel waveguides terminate along a common input/output facet of the converter. The adjacent waveguides help to shape the mode of the light propagating through the main waveguide, in so doing enabling the converter to exhibit high coupling efficiency and polarization independence in the full optical communication bands (i.e., from O to L-band) by successfully tuning the mode shape at a chip facet. The trident mode converter enables efficient and robust fiber-to-chip coupling and is useful in many applications, e.g., optical neural networks, RF-photonic filters, and others.
The foregoing has outlined some of the more pertinent features of the subject matter. These features should be construed to be merely illustrative. Many other beneficial results can be attained by applying the disclosed subject matter in a different manner or by modifying the subject matter as will be described.
For a more complete understanding of the subject matter and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
The following description assumes familiarity for silicon (Si) photonics, which is the known study and application of photonic systems using silicon as the optical medium. The silicon typically is patterned into micro-photonic components (typically Si waveguides) that operate in the infrared band, most commonly at the 1.55 micrometer wavelength used in many fiber optic telecommunications system. Photonic integrated circuits are fabricated from a variety of material systems, including electro-optic crystals such as lithium niobate, silica on silicon, Silicon on insulator, silicon nitride (SiN) as well as various polymers and semiconductor materials. While silicon photonics provide many useful properties, the compatibility of this platform with optical fiber components is still limited. This is because of the large size mismatch between the optical fiber and the silicon photonic waveguide modal distribution. The trident mode converter of this disclosure addresses the problem of how to efficiently couple light to and from the silicon photonic components.
To this end, and with reference now to
In this preferred embodiment, a maximal overlapping integral of mode area with the optical fiber is attained, preferably by tuning the geometry of the waveguides, thereby enabling a proper mode shape at the facet 108. The coupling efficiency of a mode converter is calculated (equation (1) below) from the overlap integral between an optical fiber mode and a mode at the converter tip near the chip facet:
where Ef(x,y) and Ew(x,y) are the electric field profiles of the fiber and converter tip waveguide, respectively. Minimizing fiber-to-chip coupling loss involves engineering the waveguide mode shape to match the fiber mode.
For example, and with reference to
If not addressed, the difference in refractive index for the two orthogonal polarizations (i.e., birefringence) can induce polarization dependent behavior for an optical waveguide, such as the SSC. To reduce the modal birefringence, Δneff=neff_TM−neff_TE, there are two typical approaches used, namely, by changing either the properties of the material, or the geometry of the waveguides. The design of the trident mode converter leverages the second approach. In particular, and as has been described, in one embodiment the converter comprises a SiN polarization independent waveguide geometry having a cross section of 0.4 μm×0.4 μm, and the two side waveguides are positioned (formed) close to the center waveguide with appropriate Wcenter, Wside, and Wspacing parameters. In this way the minimal modal birefringence of ˜0.001 can be attained. Contour plot 400 depicts the result, namely, equal coupling efficiencies with fiber for the TE and TM modes (i.e., polarization independence).
As depicted in
As also depicted in
While the trident configuration described is preferred, it is not intending to be limiting. The mode shaping provided by the adjacent waveguides may instead be carried out using just a single adjacent waveguide, and that single adjacent waveguide may also be configured above or below the main waveguide. Further, the converter may be configured to use three or more adjacent waveguides, e.g., waveguides situated to the left, right, above and/or below the main waveguide, or any combination (of the adjacent waveguides). In addition, although the above-described and illustrative embodiment (e.g., converter 100, in
As the above-described plots illustrate, the mode converter of this disclosure simultaneously provides low coupling loss, polarization independence, and broad bandwidth operation. The particular application(s) for the mode converter are varied and include, without limitation, optical neural networks, RF-photonic filters, and others.
The particular nature of the edge coupling provided by the mode converter may vary. In the typical use case, as has been described and depicted, the mode converter couples a photonic chip to an optical fiber. Other use cases include, without limitation, passive chip-to-passive chip coupling, passive chip-to-active chip coupling, and the like.
Claims
1. A silicon photonic-to-optical fiber mode converter, comprising:
- a supporting silicon structure;
- a main waveguide having an inverse taper structure wherein a width of the waveguide gradually reduces along a direction of light propagation to a small value at an end tip thereof; and
- a pair of adjacent waveguides positioned on opposed sides of the main waveguide, each adjacent waveguide having a width that gradually increases along the direction of light propagation to a larger value at an end tip thereof;
- the main waveguides and the pair of adjacent waveguides terminating along a common facet that is configured to be juxtaposed with an end facet of an optical fiber;
- the adjacent waveguides being configured to shape a mode of the light propagating through the main waveguide along the direction of the light propagation in a low-loss, polarization-independent, and broad band manner.
2. The mode converter as described in claim 1 wherein the supporting main waveguide is a silicon nitride (SiN) polarization independent waveguide.
3. The mode converter as described in claim 1 providing low loss coupling of less than 1 dB and low polarization dependent loss less than 0.3 dB at wavelengths at or above 1550 nm.
4. The mode converter as described in claim 1 providing one of: low loss TE coupling of less than 1.2 dB, and TM coupling loss of less than 2.6 dB, and low polarization dependent loss less than 1.4 dB over a bandwidth range over approximately 350 nm.
5. The mode converter as described in claim 1 wherein the main waveguide has a width Wcenter and each adjacent waveguide has a width Wside, and wherein an edge-to-edge spacing between the main waveguide and each adjacent waveguide is Wspacing.
6. The mode converter as described in claim 5 wherein Wcenter (Wside) is in the range of 0.1˜0.13 (0.08˜0.11) μm, respectively.
7. The mode converter as described in claim 1 wherein the main waveguide and the optical fiber have an improved misalignment tolerance relative to a single tapered waveguide output.
8. The mode converter as described in claim 1 wherein at least one adjacent waveguide is fabricated in a plane that is distinct from a plane of the main waveguide.
9. The mode converter as described in claim 1 wherein the inverse taper structure of the main waveguide is linear or non-linear.
10. The mode converter as described in claim 1 wherein an adjacent waveguide has a taper that is one of: linear and non-linear.
11. An optical light mode converter, comprising:
- a supporting silicon structure;
- a main waveguide having an inverse taper structure wherein a width of the waveguide reduces along a direction of light propagation;
- at least one adjacent waveguide;
- the main waveguide and the at least one adjacent waveguide terminating along a common facet configured to be juxtaposed with an edge structure external to the supporting silicon structure;
- the at least one adjacent waveguide configured to adiabatically shape a mode of the light propagating through the main waveguide along the direction of the light propagation in a low-loss, polarization-independent, and broad band manner irrespective of a given misalignment tolerance between the waveguide and the edge structure external to the supporting silicon structure.
12. The optical light mode converter as described in claim 11 further including at least other adjacent waveguide, wherein the main waveguide and the adjacent waveguides comprise a trident structure.
13. The optical light mode converter as described in claim 11 wherein the main waveguide is a silicon nitride (SiN) polarization independent waveguide.
14. The optical light mode converter as described in claim 11 providing low loss coupling of less than 1 dB and low polarization dependent loss less than 0.3 dB at wavelengths at or above 1550 nm.
15. The optical light converter as described in claim 11 providing one of: low loss TE coupling of less than 1.2 dB, and TM coupling loss of less than 2.6 dB, and low polarization dependent loss less than 1.4 dB over a bandwidth range over approximately 350 nm.
16. The optical light mode converter as described in claim 11 wherein the main waveguide has a width Wcenter and the adjacent waveguide has a width Wside, and wherein an edge-to-edge spacing between the main waveguide and the adjacent waveguide is Wspacing.
17. The optical light mode converter as described in claim 16 wherein Wcenter (Wside) is in the range of 0.1˜0.13 (0.08˜0.11) μm, respectively.
18. The optical light mode converter as described in claim 11 wherein the adjacent waveguide is fabricated in a plane that is distinct from a plane of the main waveguide.
19. The optical light mode converter as described in claim 11 wherein the inverse taper structure of the main waveguide is linear or non-linear.
20. The optical light mode converter as described in claim 11 wherein the adjacent waveguide has a taper that is one of: linear and non-linear, or no taper.
21. A mode converter, comprising:
- a supporting silicon structure;
- a main waveguide having an inverse taper structure wherein a width of the waveguide reduces along a direction of light propagation;
- first and second adjacent waveguides formed on opposite sides of the main waveguide and each having a width that that increases along the direction of light propagation;
- the main waveguide and the first and second adjacent waveguides terminating along a common facet;
- the adjacent waveguides configured to adiabatically shape a mode of the light propagating through the main waveguide along the direction of the light propagation in a low-loss, polarization-independent and broad band manner irrespective of a given misalignment tolerance between the main waveguide and any edge structure coupled to the main waveguide.
Type: Application
Filed: Sep 19, 2019
Publication Date: Mar 25, 2021
Patent Grant number: 11215755
Applicant: GenXComm, Inc. (Austin, TX)
Inventors: Ke Liu (Austin, TX), Thien-An Nguyen (Austin, TX)
Application Number: 16/576,169